Regulation of lipogenesis in vivo by glucose availability and insulin secretion in maternal and foetal tissues during late gestation in the rat. Effect of glucose intubation, streptozotocin-induced diabetes and starvation

Restoration of direct pathway glycogen synthesis flux in the STZ-diabetes rat model by insulin administration

Type 1 diabetes subjects are characterized by impaired direct pathway synthesis of hepatic glycogen that is unresponsive to insulin therapy. Since it is not known whether this is an irreversible defect of insulin-dependent diabetes, direct and indirect pathway glycogen fluxes were quantified in streptozotocin (STZ)-induced diabetic rats and compared with STZ rats that received subcutaneous or intraperitoneal insulin (I-SC or I-IP). Three groups of STZ rats were studied at 18 days post-STZ treatment. One group was administered I-SC and another I-IP as two daily injections of short-acting insulin at the start of each light and dark period for days 9-18. A third group did not receive any insulin, and a fourth group of nondiabetic rats was used as control. Glycogen synthesis via direct and indirect pathways, de novo lipogenesis, and gluconeogenesis were determined over the nocturnal feeding period using deuterated water. Direct pathway was residual in STZ rats, and glucokinase activity was also reduced significantly from control levels. Insulin administration restored both net glycogen synthesis via the direct pathway and glucokinase activity to nondiabetic control levels and improved the lipogenic pathway despite an inefficient normalization of the gluconeogenic pathway. We conclude that the reduced direct pathway flux is not an irreversible defect of insulin-dependent diabetes.

Perinatal development of hepatic cholesterol synthesis in the rat

Rates of cholesterol synthesis and HMG CoA reductase activity in rat liver, have been reported to be high before and low after birth. The timing of the decline in perinatal rates of cholesterol synthesis, however, is uncertain. These studies, therefore, determined in vivo rates of cholesterol synthesis using [3H]water and hepatic reductase activity in vitro in perinatal rats. The lipid composition of the plasma, liver and its microsomal subfraction were also determined. Reductase activity increased during late gestation, remained high immediately after birth, then decreased with the commencement of suckling. Rates of cholesterol synthesis increased from gestation day 18 to 20, but in contrast to reductase activity, decreased on the day before birth. Plasma cholesterol and triacylglycerol levels increased to gestation day 19, then decreased to term. By the 6th h after birth, plasma and liver cholesterol and triacylglycerol levels had increased markedly. By 48 h after birth, the high hepatic cholesterol content was associated with an increase in the cholesteryl ester fraction. The microsomal cholesterol/phospholipid molar ratio decreased from gestation day 16 until 12 h after birth, then increased markedly from 36 to 48 h. There was an apparent inverse relationship between the change in microsomal cholesterol/phospholipid molar ratio and the fatty acid unsaturation index from gestation day 16 to 36 h after birth. The results suggest that in late gestation and before suckling, the low in vivo rate of hepatic cholesterol synthesis may not be due to low activity of HMG CoA reductase.

Role of reversible phosphorylation in mediating changes in the activity of hepatic 3-hydroxy-3-methylglutaryl-CoA reductase during pregnancy and lactation in the rat

1. The expressed and total (completely dephosphorylated) activities of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase were measured in microsomal fractions isolated from cold-clamped liver samples from female rats in various stages of the reproductive cycle. 2. There was little change in total HMG-CoA reductase activity during pregnancy and early lactation, but after 2 days post partum there was a marked increase in total activity. 3. The expressed/total activity ratio of HMG-CoA reductase showed a profound decrease during the last 2 days of pregnancy. The fraction of the enzyme in the active form increased progressively during the first 2 days of lactation. 4. The combined effect of these changes was that the expressed activity of HMG-CoA reductase changed in parallel with the known changes in the hepatic rate of cholesterogenesis during pregnancy and lactation in vivo.

Regulation of rat foetal lipogenesis in brown adipose tissue in vivo and in isolated brown adipocytes during the last day of, and after prolonged, gestation

Rates of lipogenesis in foetal isolated brown adipocytes from 22-day-pregnant rats were significantly increased by lactate plus pyruvate as major substrates in the incubation medium, in comparison with the endogenous rates. Insulin stimulated foetal brown-adipocyte lipogenesis, and adrenaline or noradrenaline and isoprenaline decreased lipogenesis. Glucagon had no effect on the lipogenic rate in brown adipocytes. Progesterone administration to the mother significantly increased the rates of lipogenesis in brown adipose tissue and in isolated brown adipocytes from 22-day foetuses. Prolongation of gestation by progesterone to day 23 decreased the rates of brown-adipose-tissue lipogenesis in vivo and in isolated cells in the post-mature foetuses.

The role of prolactin and progesterone in the regulation of lipogenesis in maternal and foetal rat liver in vivo and in isolated hepatocytes during the last day of gestation

The administration of progesterone on day 21 of gestation increases the rates of lipogenesis in the liver in vivo and in hepatocytes isolated from rats on day 22 of pregnancy. Bromocriptine administration increases the rates of hepatic lipogenesis in vivo, but has no effect on lipid synthesis in hepatocytes under the same treatment conditions. Concurrently, the administration of progesterone or bromocriptine on day 21 to the mother increases the rates of lipogenesis in the foetal liver in vivo on day 22. The rates of lipid synthesis in foetal isolated hepatocytes are increased by progesterone administration, but remain unchanged by bromocriptine.

Regulation of lipogenesis and of non-saponifiable-lipid synthesis in vivo at birth and after prolonged starvation in the newborn rat

The rate of lipogenesis in the liver was increased by glucose injection at birth, mediated by the insulin secretion. In addition, glucagon decreased the rates of lipogenesis and non-saponifiable-lipid synthesis after birth. These rates decreased after prolonged starvation in the newborn rat. Tri-iodothyronine injection increased the rates of lipogenesis enhanced in response to glucose administration after prolonged starvation in liver and brown adipose tissue. Dexamethasone, however, increased the rates of lipogenesis enhanced in response to glucose in liver and prevented the increase in the rates of lipogenesis in brown adipose tissue.